1) Field of the Invention
The present invention relates to a process and an apparatus for continuously producing polyester type polymers such as polybutylene terephthalate and polyethylene terephthalate.
2) Related Art
Since polyethylene terephthalate (hereinafter referred to as PBT) resins are excellent in the crystallizing characteristic and also excellent in mechanical properties, electric characteristics and heat resistance, they have been used for applications such as electric machines, electronic parts, mechanical parts and automobiles and their demand has been increased steadily.
Heretofore, for the general PBT production process, a terephthalic acid alkyl ester comprising dimethyl terephthalate as a main ingredient and a glycol comprising 1,4-butanediol (hereinafter referred to as BD) as a main ingredient are placed at an appropriate ratio in a mixing vessel, a transesterification catalyst is added and conditioned and then they are sent to a transesterification reaction vessel set to a predetermined reaction temperature by a pump. In the transesterification reaction, two or three stirring vessels with stirring blades are disposed in series and methanol formed as reaction by-products, and tetrahydrofuran (hereinafter referred to as THF) formed by decomposition of the methanol formed as reaction by-products and BD and water are separated in a distillation tower. Then, a polymerization catalyst is added and the process proceeds to the polymerizing reaction step. At first, vertical stirring vessels or horizontal stirring vessels are disposed in plurality for the prepolymerization step and, further, a horizontal stirring vessel is disposed as a final polymerization step. In the vessels for the polymerization step, a condenser is disposed for removing BD, THF and water formed as reaction by-products and the step is operated in a reduced pressure atmosphere at high temperature. In the existent continuous PBT reaction step, the number of the reaction vessels is 4 to 6, a stirring blade and a power source therefor are installed in each of the reaction vessels, and a distillation column or a condenser for separating and removing the reaction by-products are disposed.
In these production steps described above, since the products are exposed in a reaction state at high temperature for a long time, a portion of the resin formed by polymerizing reaction is broken at the bond sites by thermal decomposing reaction to lower the degree of polymerization and increase the acid value of the resin (carboxyl group concentration at the terminal end of the polymer), to worsen the quality. Further, since the polymerization step is operated in an reduced pressure atmosphere, it is necessary that the evacuation unit has to be operated by an additional device, and high maintenance cost and high installation cost are required for the operation of the apparatus.